A neuropsychological profile was plotted for individuals with Alzheimer's (AD), Huntington's (HD), or Parkinson's (PD) disease. The evaluations extended into memory, learning and perception, utilizing standard and experimental tasks, also establishing normative references for functional changes accompanying the aging processes. The results revealed common as well as specific deficits, implicating involvement of different brain structures. Specifically, AD is accompanied by marked deficits in selective attention and episodic memory, and visuospatial disturbances; there were few qualitative differences between demented and age-matched subjects. These data indicate that Alzheimer's patients may be unable to encode material. AD and HD patients showed pronounced but dissimilar deficits with visuospatial and constructional tasks. The behavioral data extend neuropathologic impressions of degeneration of the frontal striatal system in HD and temporo-parietal, cortical involvement in AD. With PD, performance decrements were less prominent and many patients continue to function at an unimpaired level; dysfunctioning varied in relation to complexity and executive requirements, which aligned strongly with fronto-striatal changes. Unlike HD, PD patients usually showed fewer behavioral and personality changes; emotional expression was not one of disinhibition.